Operating devices for a lamp are nonlinear since they have a combination of a rectifier and a downstream driver circuit, which is in the form of an inverter or AC-to-DC converter and which is used to operate the lamp. In addition, the characteristic of the lamp is often nonlinear, wherein this applies, for example, to light-emitting diodes or else gas discharge lamps, in particular fluorescent lamps. Accordingly, even in the case of such electronic control gear or other operating devices for lamps, power factor correction circuits are often used, wherein this is also recommended since the permissible return of harmonics into the supply system is regulated by standards.
Power factor correction (PFC) is used to eliminate or at least reduce harmonic currents in an input current. Harmonic currents can occur in particular in the case of nonlinear consumers, such as, for example, rectifiers with downstream smoothing in switched mode power supplies, since, in the case of such consumers, the input current is phase-shifted despite the sinusoidal input voltage and is distorted non-sinusoidally. The relatively high frequency harmonics occurring in the process can be counteracted by an active or clocked power factor correction circuit connected upstream of the respective device since the power factor correction eliminates the nonlinear current consumption and shapes the input current such that it is substantially sinusoidal.
A circuit topology which is based on a boost converter, which is also referred to as a step-up converter, is often used for power factor correction circuits. In this case, an inductance or coil, to which a rectified AC voltage is supplied, is charged with an input current or discharged during switch-on/switch-off of a controllable switch. The discharge current of the inductance flows via a diode to the output, which is coupled to an output capacitance, of the converter, with the result that an increased DC voltage in comparison with the input voltage can be tapped off at the output. Likewise, however, other types of converters are also conventional in power factor correction circuits, such as flyback converters or buck converters, for example.
Such a power factor correction circuit can be operated in various operating modes, which are described with reference to a boost converter in “Control Techniques For Power Factor Correction Converters”, L. Rosetto, G. Spiazzi, P. Tenti, Proc. of PEMC 94, Warsaw, Poland, pp. 1310-1318, 1994, by way of example. In particular, operation with a continuous current by means of the abovementioned inductance (so-called “continuous conduction mode”, CCM), operation with a discontinuous inductance or coil current (“discontinuous conduction mode”, DCM) or operation in the limit range between continuous and discontinuous current through the inductance (“borderline conduction mode” or “boundary conduction mode”, BCM) is known.
Thus, for example, during BCM operation, each time the coil current drops to zero during the discharge phase of the coil this is taken as a cause for starting a new switching cycle and switching on the switch again in order to recharge the coil. During DCM operation, on the other hand, once the coil current has dropped to zero during the discharge phase, first a predetermined additional time is waited until the switch is closed again.
In respect of the further details relating to the individual known operating modes, reference is made to the full content of the abovementioned publication.
The individual operating modes have various advantages, with the result that changeover often takes place between the respective operating modes depending on the operating conditions of the power factor correction circuit during operation.
In this case, there is, in principle, the problem of controlling the transition between the individual operating modes reliably and using simple means.